An operando Raman study of structure and reactivity of alumina-supported molybdenum oxide catalysts for the oxidative dehydrogenation of ethane

The structural and catalytic properties of supported MoO3/Al2O3 catalysts with Mo surface densities, n(s), in the range 1.1-12.5 Mo/nm(2) were studied for the oxidative dehydrogenation of ethane by in situ Raman spectroscopy with simultaneous catalytic measurements at temperatures of 400-550 degrees C. Isolated mono-molybdates (MoO4) and polymolybdates are formed at various proportions (depending on the loading) on the catalyst surface under dehydrated conditions; bulk Al-2(MoO4)(3) crystals are formed at ns exceeding the monolayer. Under reactive environment (C2H6/O-2), the Raman features attributed to Mo=O and Mo-O-Mo modes appear to be perturbed, indicating that surface MoOx species are reduced to lower valence states. The combined information on catalyst structure under reaction conditions and catalytic data shows a concurrence between the trends for activity per Mo (turnover frequency [TOF]) and for the number of Mo-O-Al bonds per Mo as a function of ns. The catalytic activity is related to the anchoring Mo-O-support bonds. Structure-activity/selectivity relations are derived and discussed. (c) 2006 Elsevier Inc. All rights reserved.